Array Signal Processing for Communications from High Altitude Platforms and Other Applications

Digital beamforming (DBF) technology using antenna array has reached a sufficient level
of maturity that it can be applied to communications to improve system capacity. In this
thesis, the applications to digital beamforming in the communications from high altitude
platforms (HAPs) and the high data rate OFDM signal transmission in underwater
acoustic channel are respectively investigated.

Both conventional and adaptive beamforming methods are investigated in the application
to HAP communications. A three-step footprint optimization method based
on planar array and hexagonal cellular structure is proposed to generate arbitrary
beampattern footprints. A vertical antenna array forming ring-shaped cells is shown to
improve the coverage performance and reduce system complexity. In the adaptive beamforming
scenario, the minimum variance distortionless response (MVDR) beamformer is
investigated. We show that the robustness of the MVDR beamformer can be improved by
applying the Mailloux’s null-broadening method in the downlink scenario and propose a
modified constrained optimization method, which further improves the performance of
Mailloux’s method. Finally, a method combining cellular and null-broadening adaptive
beamforming is proposed. Simulation results show that the method achieves better
coverage performance for cellular communications than when using the same number of
aperture antennas. Its advantages in payload reduction are even more significant.

The adaptive beamforming techniques are also applied in the receiver design for
high data rate OFDM signal transmission in the fast-varying underwater acoustic channel.
This work is based on experimental data obtained in the Pacific Ocean at a distance
of 30 km in 1989. Several space-time signal processing techniques are investigated. A
variety of direction of arrival (DoA) estimation methods are considered, which are based
on signal interpolation and the modified MVDR beamformer. The DoA estimates are
used for angle-separation of signals for accurate time-delay compensation. The OFDM
signals from different directions are linearly combined after Doppler compensation and
channel equalization. An adaptive Doppler filter is applied in frequency domain for
removing residual intersymbol interference. The null-broadening beamforming method, originally proposed in the HAPs communications, is applied to improve the robustness
of the MVDR beamformer. Experimental results show that the adaptive beamforming
based space-time techniques are efficient and provide the bit-error-rate of 10¡3 and 10¡2
respectively for the data efficiency 0.5 bit/s/Hz and 1 bit/s/Hz.